The present invention generally relates to an electrical filter and more particularly, to a piezoelectric filter of a three terminal type superior in spurious characteristics.
Referring to FIGS. 1 and 2 showing a prior art piezoelectric filter element of three terminal type (referred to as merely a three terminal type piezoelectric filter hereinbelow) utilizing the breathing or expansion mode vibration of a rectangular plate, the known piezoelectric filter F generally comprises a piezoelectric ceramic base plate or substrate B in a configuration of a parallelogram, an input electrode Ei also in a configuration of a parallelogram formed at approximately a central portion of one main surface of the piezoelectric substrate B, an output electrode Eo formed along the peripheral edges of the one main surface of the substrate B so as to surround the corresponding peripheral edges of the input electrode Ei, and a common electrode Ec formed on the other main surface of the substrate B.
For reference, there is shown in FIG. 3 an example of a circuit employing one element of the three terminal type piezoelectric filter F as described above, in which circuit, an AM-front end C1 is coupled to an AM detection circuit C2 through an intermediate frequency transformer IFT and the piezoelectric filter F, while spurious characteristics upon combination of the intermediate frequency transformer IFT and the piezoelectric filter F as in FIG. 3 are represented in a graphical form in FIG. 4.
As is seen from FIG. 4, in the conventional three terminal type piezoelectric filter F as described with reference to FIGS. 1 and 2, a spurious response due to the edge mode vibration is noticed in the vicinity of 650 KHz, and thus, there have been cases where troubles or interferences due to "beat" are brought into question during reception especially in radio sets for receiving long wave band regions.